Abstract
To explain the cholesterol-reducing effects of dietary fibres, one of the major mechanisms proposed is the reduced reabsorption of bile acids in the ileum. The interaction of dietary fibres with bile acids is associated with their viscous or adsorptive effects. Since these fibre characteristics are difficult to investigate in vivo, suitable in vitro methodologies can contribute to understanding the mechanistic principles. We compared the commonly used centrifugal approach with a modified dialysis method using dietary fibre-rich materials from different sources (i.e., barley, citrus, lupin, and potato). Digestion was simulated in vitro with oral, gastric, and small intestinal digestion environments. The chyme was dialysed and released bile acids were analysed by high-performance liquid chromatography. The centrifugation method showed adsorptive effects only for cholestyramine (reference material) and a high-fibre barley product (1.4 µmol taurocholic acid/100 mg dry matter). Alternatively, the dialysis approach showed higher values of bile acid adsorption (2.3 µmol taurocholic acid/100 mg dry matter) for the high-fibre barley product. This indicated an underestimated adsorption when using the centrifugation method. The results also confirmed that the dialysis method can be used to understand the influence of viscosity on bile acid release. This may be due to entrapment of bile acids in the viscous chyme matrix. Further studies on fibre structure and mechanisms responsible for viscous effects are required to understand the formation of entangled networks responsible for the entrapment of the bile acids.
Highlights
Hypercholesterolemia is one of the major risk factors for coronary heart disease—the most frequent cardiovascular disease in developed and developing countries [1]
As insulin is an activator of a rate-limiting enzyme in cholesterol synthesis, this mechanism could contribute to the reduction of cholesterol [6]
First-order kinetics can be applied to the analysis of bile acid release across a dialysis membrane. This could enable the evaluation of viscous and adsorptive effects of dietary fibres based on the parameters of diffusion kinetics [16,19]. We evaluated this approach by dialysis of in vitro digested chyme containing dietary fibre from different sources and studied adsorptive as well as viscous effects under simulating conditions of the small intestine
Summary
Hypercholesterolemia is one of the major risk factors for coronary heart disease—the most frequent cardiovascular disease in developed and developing countries [1]. Threapleton et al [2] have demonstrated the inverse correlation between coronary risk factors and an increased dietary fibre intake. One hypothesis is the impairment of fat and cholesterol absorption and their increased secretion induced by the interaction with dietary fibres or the inhibition of relevant enzymes [4]. As insulin is an activator of a rate-limiting enzyme in cholesterol synthesis, this mechanism could contribute to the reduction of cholesterol [6]. Another hypothesis is related to the bacterial fermentation of dietary fibres to produce short chain fatty acids, which could inhibit the hepatic cholesterol synthesis
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